Vehicles equipped with pneumatic tires have come under increased scrutiny as requiring more detailed information regarding tire pressure. Such tire pressure information is required for safe operation of the vehicle and for greater fuel efficiency (in the case of motor vehicles) since underinflated vehicle tires cause increased fuel consumption compared to properly inflated tires. To monitor tire pressure, various systems have been proposed. In general, tire pressure monitoring systems include a pressure monitor and an element which can transmit the tire pressure information, typically in the form of a radio frequency (r.f.) signal, to a receiver. Such receivers can be positioned in a monitoring station (for tire-based machinery such as rubber-tired gantry systems used in container shipment systems) or inside a vehicle for automobiles and trucks. In this manner the tire pressure information can be displayed and/or alarms can be set to alert the operator of the machinery/vehicle when one or more tires has reached a critical level of underinflation or overinflation.
Several configurations have been proposed. In one system, part of the pressure monitoring system is the tire valve itself, such as the valve stem or the valve pin as an antenna or an antenna portion to transmit a signal from an internal tire pressure sensor to a receiver. Such systems are disclosed in U.S. Pat. Nos. 6,101,870, 7,059,178, 7,328,609, 7,549,329 and U.S. Patent Publication 2008/0121031. However, for high pressure tires, such as the gantry equipment discussed above, valve based systems have can have air leakage problems and valves can be easily damaged.
In another configuration, a tire pressure sensor can be mounted on a tire rim (for tubeless tires). However, there are numerous custom rim designs in the wide variety of machines that employ pneumatic tires. Therefore, it is difficult to install rim-mounted tire pressure sensors on all the types of custom rims that are used in such tires. For tires that cannot use rim-mounted pressure sensors, pressure sensors are typically affixed to an inner tire wall using, for example, special adhesives. Such mounting is time-consuming and also wasteful of the tire sensor which is discarded at the end of the useful tire life.
It has been proposed to use freely-moving pressure sensors inside tires that include piezoelectric elements for energy harvesting from vibrations during vehicle motion. Such systems are described in U.S. Patent Publication 2010/0164705 and U.S. Pat. No. 7,936,109. While such sensors may have application for relatively fast-moving vehicles (such as automobiles) which generate large numbers of collisions, many mechanical systems with pneumatic tires are slow-moving or do not move for long periods. Therefore, insufficient numbers of collisions are generated to power such sensors. Even when sufficient collisions are generated, there are concerns that a sensor can suddenly fail during a sensor collision. For personal automobiles, a sensor failure is not catastrophic; however in large equipment systems, replacement of a sensor can mean closing down an entire system while a new sensor is installed.
Therefore, there is a need in the art for freely movable pressure sensors positioned within tires that are easily installed, reusable, and reliable.